Clarify that PromptResponse.usage and UsageUpdate.cost are expected for all servers #890

Problem

The ACP schema defines PromptResponse.usage and UsageUpdate.cost as optional fields, but there's no guidance that servers should populate them. This has led to divergent implementations:

Impact

ACP clients that rely on the standard fields for usage tracking need per-server fallback logic when some servers put data in non-standard locations (_meta). This defeats the purpose of a standardized protocol.

Suggestion

Consider documenting (or making normative) that ACP servers:

1. SHOULD populate PromptResponse.usage with input_tokens, output_tokens, and total_tokens
2. SHOULD send UsageUpdate notifications with a cost field when monetary cost data is available

This would make usage/cost tracking work consistently across all ACP servers without client-side workarounds.

Reference

• Gemini CLI puts token data in _meta.quota.token_count instead of usage: google-gemini/gemini-cli#24280
• Claude Agent ACP reference implementation: populates both fields (source)